(Like a roof or ac repair would be my example.) maybe a lender will chime in to see if even something like that could be done.

Signs like tarps on roofs, black/brown pools, and cars parked all over the yard. 

An older roof?

Given what I know it needs without even seeing it, though (new roof, fascia repairs, animal control, new boilers, lead and asbestos remediation, oil tank removal--and that's without knowing anything about it structurally or cosmetically on the inside), I'm guessing I would have to put in roughly $100k in rehab.

Roof, windows, foundation, siding, driveway, mechanicals. 

The the sellers can properly make an educated decision, but then how many people would want to buy a house where the foundation is end of life and to fix it is a 20k expense.The void wall is not that big of a deal as long as the home can properly cool or heat, which I believe just roof insulation and the right tonnage will accomplish. 

As time goes you need to establish systems and processes for smooth transactions.

$100k only got 1/4 of the roof, and site work will be about that as well.

The listing agent reached back out to my realtor and asked if we were still interested and that the seller was now offering a credit to buyer to pay for a new roof to be placed on the house and garage so we made another offer and got under contract.

If you have to replace a roof, that will take up a years worth of your free cashflow.